[FR] As a user viewing a userstory I would like to hide completed subtasks

[mbautista]

Hello,
First thank you very much for Taïga ! It's a really good software and we love it :)

Please describe the problem / need you are trying to solve.

When creating a user story with an important list of tasks, the list become hard to read when there are lots of completed tasks with only a few remaining undone tasks.

Describe the feature or the improvement you'd like and what are you trying to achieve.

We would like to add a "Show/hide completed tasks" at the top of the task list in order to toggle these tasks.

Thank you very much.
Mathieu.